This invention relates to tack-free polymer pellets, for example, ethylene/vinyl acetate copolymer pellets having good handling and block resistance characteristics.
For the purpose of this invention, the term "vinyl acetate copolymer" includes both the dipolymers and the terpolymers of ethylene with vinyl acetate and with carbon monoxide.
Ethylene/vinyl acetate (EVA) copolymers find wide commercial use in flexible, heat sealable, functional, and decorative hot-melt and solvent-applied coatings; as wax additives to impart toughness, flexibility, and adhesion; as blending resins to impart flexibility to brittle materials; and as a component of hot-melt and solvent-applied adhesives. Most commercial EVA dipolymers contain about 2-55% by weight of vinyl acetate. Terpolymers of ethylene with vinyl acetate and with carbon monoxide may contain about 18-40 weight percent of vinyl acetate and 2-12 weight percent of carbon monoxide. Dipolymers of ethylene with vinyl acetate are available, e.g., from E. I. Du Pont de Nemours and Company, Wilmington, Del., under the trademark "Elvax"; the terpolymers with carbon monoxide can be made according to the teachings of U.S. Pat. Nos. 2,495,286 to Brubaker and 3,780,140 to Hammer. As the vinyl acetate content of the copolymer increases, the copolymer tends to become increasingly sticky. Copolymer pellets, the usual commercial form, at the same time increasingly tend to block, that is, to stick together and form large agglomerates, especially at somewhat elevated temperatures and under pressure of their own weight. This stickiness and the resulting blocking also increase with the copolymer's melt index, that is, with decreasing molecular weight. For example, for bulk EVA copolymer containing about 25-80% by weight of vinyl acetate, problems of this nature usually are encountered when the melt index exceeds 6 g/10 min. Yet, even those copolymers will have different flow properties and blocking tendencies, depending on their melt index values. For example, those copolymers having melt indices below about 50 g/10 min can be shipped in 26.6 kg bags or in 500 kg boxes without blocking. Those having melt indices above about 100 g/10 min cannot be shipped in large containers without risk of blocking but can be shipped in bags. Pellets of all these copolymers are free flowing and can be handled without difficulty. Higher vinyl acetate content copolymers, for example, those containing above 35 weight percent of vinyl acetate, are very sticky, have poor flow properties, and block even in regular size bags. Most higher vinyl acetate content EVA copolymers cannot be shipped in hopper cars. Other polymers which present difficulties of similar nature, but especially in shipping and storage, are copolymers of ethylene with methacrylic acid in which the carboxyl groups are either free or partly neutralized with metal ions, copolymers of ethylene with methyl methacrylate, and terpolymers of ethylene with n-butyl acrylate and carbon monoxide. Copolymers of ethylene with methacrylic acid are available from E. I. du Pont de Nemours and Company under the name "Nucrel". The partly neutralized, ionic form polymers (ionomers) are available from the same source under the name "Surlyn". Copolymers of ethylene with methyl methacrylate can be made according to U.S. Pat. Nos. 3,287,335 to Stuetz, 3,658,741 to Knudson et al., and 3,949,016 to Agouri et al. Terpolymers of ethylene with n-butyl acrylate and carbon monoxide can be made following the techniques of the above Brubaker and Hammer patents.
A particularly serious problem, which occurs in the pelletization and packaging equipment handling tacky polymers, especially certain grades of ethylene/vinyl acetate copolymers, is the tendency of the pellets to stick both to themselves and to the equipment, thus slowing down the process flow rate below its optimum value. If the pellets cannot be handled at normal process speeds or cannot be handled at all, the efficiency of the process suffers greatly.
All polymers contemplated by this invention have a tendency to block in storage in pellet form but not all present significant handling difficulties or have poor flow properties. Poor flow properties are indicated by a low "stick temperature", which is determined in a standardized test developed by E. I. du Pont de Nemours and Company and described below. This test measures the tendency of polymer pellets to flow after holdup for a fixed time at a fixed temperature and pressure. The EVA copolymers of interest in the practice of the present invention have a stick temperature of at most about 40.degree. C., while the other polymers of interest have a stick temperature of less than about 60.degree. C. Polymers having stick temperatures above those temperatures do not usually have a tendency to block under normal handling and storage conditions. Various techniques have been proposed to improve the flow properties of EVA copolymer pellets and to reduce their tendency to block, especially, coating the pellets with various materials such as clay, talc, or powdered polyethylene. See, for example, U.S. Pat. No. 3,528,841 to Donaldson et al. These methods are effective in varying degrees but have a shortcoming in that they do not always completely eliminate agglomeration of the pellets, which may occur before the coating is applied; and, further, the coating may not remain indefinitely attached to a pellet but may be lost in part in normal handling.
Slip (anti-tack) agents are offered commercially to improve the slip of polymer films and polymer processability. These include, for example, various fatty acid amides, which for the most part have been found to be ineffective or only marginally effective in reducing the tendency of EVA pellets to block but are quite effective in certain polymers of other types.